This invention is directed to an analytical method which permits quick and accurate repetitive determinations of oxygen concentrations in gas streams.
Oxygen often is found in gaseous emissions from various industrial processes, especially those involving combustion, as well as in some chemical process off-gases, whenever air is injected into the process stream. Although the presence of oxygen in those emissions is not considered harmful to the environment, air acts as a diluent for various pollutants present in those emissions so that it is impossible to ascertain compliance with environmental control regulations unless this dilution level can be accurately determined. It thus is necessary to be able to accurately monitor the level of oxygen in gaseous industrial emissions. An accurate measurement of oxygen level is also invaluable for combustion control. A typical source of gas emissions is a refinery stack, the concentration of oxygen in stack gases being anywhere from negligible to as much as 20% by volume or more.
Various oxygen monitoring devices are commercially available. For example, an electrochemical device based on zirconium oxide can be used, but it has the drawback of requiring an operating temperature of at least 700.degree. C., above the ignition point of most combustible gases, such as refinery gases. This requires isolation of the device in an explosion-proof box, which adds to the bulk, cost, and inconvenience.
Another oxygen-monitoring method is based on paramagnetic property measurements. This operation is carried out at a room temperature, so that it is necessary to use complicated sampling systems to avoid errors due to moisture condensation.
Other methods rely on color changes of solutions wherein the active component undergoes oxidation from a leuco form to a colored form. Such reactions, although well known and routinely used in oxygen analysis, are not practical for rapid and repetitive oxygen determination.
A rapid and possibly useful method of monitoring oxygen concentrations in gas streams is described in U.S. Pat. No. 2,389,046 to D. G. C. Hare, granted in 1945. According to that disclosure, a gas sample is diverted and continuously passed through two optical cells illuminated by the same light source. Nitric oxide is introduced into one of the cells in sufficient amount to react with all of the oxygen in the gas sample, at least some of the reaction product being "the red nitrogen tetroxide, N.sub.2 O.sub.4." The intensities of light transmitted by the optical cells are continuously detected by two photoelectric cells connected in a bridge circuit, and the difference is measured by a galvanometer. This method is said to require only 5 to 10 seconds for a complete color development.
It is believed that the patentee actually measures the light absorption due to nitrogen dioxide, NO.sub.2, rather than tetroxide, N.sub.2 O.sub.4, which is known to be colorless. However, assuming good calibration of the equipment and maintenance of constant flows and temperature, there is no reason why this method should not be able to function satisfactorily. However, it has not found wide acceptance in the industry. The main reasons for this are believed to be as follows: The gas flows of the sample, and of nitrogen oxide, NO, must be known and controlled exactly because NO acts as a diluent for the gas sample to which it is added, and the galvanometer reading must be corrected for this dilution. Since the rate of reaction (1), above, is very much temperature dependent (the rate decreasing with increasing temperatures), it is important to control the operating temperature closely in view of the short residence time in the optical cells. It is necessary to maintain carefully controlled, constant gas flows, both because the calibration depends on the gas flows and because the gas flows of the gas sample in both cells must be identical in order to eliminate the background effect.
Therefore, a sample and reliable method for monitoring oxygen concentrations at moderate temperatures, capable of providing accurate results in a short time, is very desirable.